Contention window adjustment methods capable of load-adaptive backoff in a network and machine-readable storage medium therefor

ABSTRACT

A contention window adjustment method capable of load-adaptive backoff (LAB) function in a network. At least one middle contention window (CW mid ) is set for a station. If the contention window (CW) of a station is greater than CW mid , then that station considers that the network traffic load is heavy. Similarly, if the CW value of a station is no more than CW mid , then that station considers that the network traffic load is light. If collisions occur when the station is transmitting data frames, CW is doubled. Once the station successfully transmits the data frame, CW is decreased by different ratios according to the network traffic load.

BACKGROUND

The invention relates to medium access controls, and more particularly,to contention window adjustment methods capable of Load-Adaptive Backoff(LAB for short) in a network.

The media access control (MAC) protocol defines data transferringmethods for stations (communication devices, such as laptops, personaldigital assistants, mobile phones, and others) in a medium-sharednetwork. The station following a carrier sense multiple access (CSMA forshort) protocol must detect whether the medium is busy before sending aframe. Networks with MAC methods based on CSMA comprise wireless localarea networks (WLAN) conforming to the Institute of Electrical andElectronic Engineers (IEEE) 802.11 standard, ultra wideband (UWB)networks conforming to IEEE 802.15.3 standard, wireless sensor networks(WSNs) conforming to the IEEE 802.15.4 standard, Ethernets conforming tothe IEEE 802.3 standard, and others. Several MAC methods, including IEEE802.3, 802.11, 802.15.3, and 802.15.4 standards, utilize the BinaryExponential Backoff (BEB) algorithm to adjust the contention window (CW)to avoid or resolve collisions.

In this invention, we take IEEE 802.11 for example to describe theoperations of BEB. IEEE 802.11 defines two access functions: one isdistributed coordination functions (DCF), and the other is pointcoordination functions (PCF). Stations complying with the wirelessfidelity (Wi-Fi) specification must support DCF. DCF employs the CarrierSense Multiple Access with Collision Avoidance (CSMA/CA) method totransmit frames. If station A wants to send a data frame to station B,then station A should sense the medium before sending a frame. However,even if the medium is idle at this moment, station A cannot immediatelytransmit a frame. Station A must wait for a period of “DCF Inter FrameSpace (DIFS)” time. After the DIFS time passing, station A can transmitdata frames. On the other hand, if the medium is busy when station Awants to transmit, then station A defers until the medium is determinedto be idle for DIFS, and then station A selects the backoff slots whosenumber is a random integer between zero and CW−1, where CW denotes thecontention window. The backoff time decreases continuously by one slot.It is noteworthy that backoff countdown process may not be alwayssuccessful. If the medium is determined busy at any time during abackoff slot, then the backoff timer should be frozen. When the channelis sensed idle again for more than a DIFS, the backoff timer can bereactivated. Whenever the backoff timer reaches zero, transmission shallcommence. The effect of this DCF procedure is that when multiplestations enter the backoff stage at the same time, then the stationchoosing the minimum backoff time will win the contention. Uponreception of the data frame, the destination station (station B) shallreply the ACK frame after an elapsed SIFS (Short InterFrame Space). Notethat SIFS<DIFS. If the sending station does not hear the ACK signal, itshall resend the data frame after waiting at least an ACK timeoutinterval or drops that frame when the DCF retry limit is reached. By the802.11 standard, the backoff time is defined as follows.BackoffTime=DUR(0,CW)*SlotTime, CWS=CW−1,where DUR(0,CW) is a function which will return an integer randomly anduniformly between 0 and CW−1. Notice that CW and SlotTime arePHY(physical layer)-specific. For example, when the PHY is DirectSequence Spread Spectrum (DSSS) defined in IEEE 802.11b, then SlotTimeis 20 μs and the possible value of CWS(CW+1) is 32, 64, 128, 256, 512,or 1024.

To resolve collisions, DCF employs BEB and its correspondingretransmission method. FIG. 1 is a schematic view of CWS adjustmentusing BEB. The values of the maximum contention window (CW_(max)) andthe minimum contention window (CW_(min)) are defined in 802.11. Noticethat we define that CWS=CW+1, CWS_(max)=CW_(max)+1, andCWS_(min)=CW_(min)+1 for convenience. Based on BEB, whenever data frametransmission fails, the value of CWS is doubled until CWS equalsCWS_(max). Once the data frame transmission succeeds, the value of CWSjumps to CWS_(min) directly.

The drawbacks of BEB are as follows. Even if a station fails to transmita data frame many times, the value of CW will reduce to CW_(min) oncethat station successfully transmit a data frame. We believe that this CWvalue (CW_(min)) is too small since the medium contention may be stillsevere. The inadequate CW value (CW_(min)) may incur more collisions,causing the throughput down. In addition, the collided station has alarger CW value, which makes it more difficult to seize the medium thana non-collided station (whose CW value is CW_(min)). In other words,collided stations have lower chance to seize the medium. Therefore, BEBcannot support short-term fairness among contending stations.

To support Quality of Service, IEEE 802.11 Task Group E is strugglingfor defining the 802.11e standard. 802.11e assigns different values ofCW_(max) and CW_(min) for different priority access categories. Inparticular, a higher priority station has smaller values of CW_(max) andCW_(min). Regardless of the priority, the station in 802.11e stillfollows the BEB to transmit data frames. This implies that theabove-mentioned problems may still occur in 802.11e.

As described, the invention discloses a contention window adjustmentmethod capable of load-adaptive backoff (LAB). Compared with BEB, LABcan appropriately adjust the value of CW (CWS) according to traffic loadin a network, significantly reducing the collision probability and datatransmission delay, thus improving throughput and fairness.

SUMMARY

Contention window adjustment methods capable of load-adaptive backoff(LAB) in a network are provided. In an embodiment of such a method, amiddle contention window (CW_(mid)) is defined and at least one stationis provided. The value of CW_(mid) is between the maximum contentionwindow (CW_(max)) and the minimum contention window (CW_(min)). When astation transmits a data frame successfully, if its contention window(CW) is greater than CW_(mid), CW is decreased by a ratio parameter p,and if its CW value is less than or equal to CW_(mid), CW is decreasedby the second ratio parameter q. The first ratio parameter p is greaterthan the second ratio parameter q. If a station fails to transmit dataframes, the value of CW should be doubled. However, Under anycircumstances, the value of CW cannot be greater than CW_(max).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples of embodiments thereof with referencemade to the accompanying drawings, wherein:

FIG. 1 is a schematic view of contention window adjustment using BEB;

FIG. 2 is a schematic view of an embodiment of contention windowadjustment using LAB; and

FIG. 3 is a flowchart of an embodiment of a contention window adjustmentmethod capable of load-adaptive backoff (LAB) in a network.

DETAILED DESCRIPTION

The invention discloses a contention window adjustment method capable ofload-adaptive backoff (LAB) in a network.

The invented load-adaptive backoff (LAB) method can appropriately adjustcontention window (CW) according to traffic load in a network, greatlyreducing collision probability and transmission delay, thereforeimproving throughput and fairness. Compared with DCF, LAB furtherdefines at least one middle contention window (CW_(mid)), except for CW,CW_(max), and CW_(min). CW_(mid) is between CW_(max) and CW_(min) forreflecting the traffic load in a network. For convenience, we let themiddle contention window size (CWS_(mid)) equal to 1+CW_(mid).

FIG. 2 is a schematic view of an embodiment of contention windowadjustment using LAB. In this embodiment, CWS_(mid) is defined as 256.In LAB, a station regards the traffic load heavy when CWS>CWS_(mid).

When a network has a heavy traffic load (CWS=1024 or 512, for example),the CWS of a station is decreased to 75% (the first ratio parameter, p)(i.e. CWS:=CWS*0.75) after a data frame is successfully transmitted.When a network has a light traffic load (CWS=128 or 64, for example),CWS of a station is decreased to 50% (the second ratio parameter, q)(i.e. CWS:=CWS*0.5) after a data frame is successfully transmitted. Thisprocess can be described using the following mathematical formulas.

CWS:=min{2*CWS,CWS_(max)}, if transmission fails;

CWS:=max{p*CWS,CWS_(mid)}, if data transmission succeeds andCWS>CWS_(mid);

CWS:=max{q*CWS,CWS_(min)}, if data transmission succeeds andCWS<CWS_(mid). Note that 0<q<p<1 and the “max” and “min” functionsreturn the maximum value and the minimum value from the setrespectively.

In this embodiment, when CWS does not equal to 32 or 64, CWS will not bereduced to CWS_(min) even if data frames are successfully transmitted.

FIG. 3 is a flowchart of an embodiment of a contention window adjustmentmethod capable of load-adaptive backoff (LAB) function in a network.

A plurality of communication devices are first provided in a network. Inother words, this network includes at least station A and station B, andthese two stations can mutually transmit data frames. In each station,the middle contention window (like CWS_(mid) as described) is set (stepS1). Station A selects backoff time randomly according to a contentionwindow (or contention window size (CWS) as described) thereof (step S2)and begins to count down the backoff time via a counter (step S3).During backoff, station A continuously monitors whether the network(medium) is busy (step S4). When the network becomes busy (for example,station B is transmitting), station A freezes the backoff counter (stepS5) and performs the carrier sense procedure continuously (step S4).

When the network becomes idle (i.e. station B stops transmitting dataframes), station A determines whether the backoff counter reaches zero(step S6), and, if so, station A can transmit a data frame (step S7); ifnot, the process proceeds to step S3. After sending the data frame,station A waits for the ACK frame. The ACK frame can be used todetermine whether that data frame is successfully transmitted (step S8).Upon failure, station A then determined whether the number ofretransmission is no less than the retry limit value (step S9). If so,station A drop that data frame. If not, CWS is doubled (CWS:=min{2*CWS,CWS_(max)}) (step S10) and then process proceeds to step S2. Note that,under any circumstances, the value of CWS should not exceed the maximumcontention window (as with the described CWS_(max)).

If station A successfully transmits the data frames, it then determineswhether CWS is greater than CWS_(mid) (CWS>CWS_(mid)?) (step S11). Ifso, the traffic load may be heavy. Thus in order to reduce collisionprobability, CWS is decreased to 75% (CWS:=max{p*CWS, CWS_(mid)}, i.e.CWS:=p*CWS and p=0.75) (step S12) and then the process proceeds to stepS2. If not, the traffic load may be light. Thus in order to reduceaverage backoff time, CWS is decreased to 50% (CWS:=max{q*CWS,CWS_(min)} i.e. CWS:=q*CWS and q=0.5) (step S13) and then the processproceeds to step S2. Note that, under any circumstances, the value ofCWS should be greater than or equal to the minimum contention window(like CWS_(mid) as described).

A contention window adjustment method of the invention can set aplurality of middle contention windows. When two middle contentionwindows are set, the described process in FIG. 3 may add two moredetermination conditions, comprising (CWS>CWS_(mid1)?),(CWS_(mid1)<CWS<CWS_(mid2)?), and (CWS<CWS_(mid2)?). When one of thedescribed conditions is met, CWS is multiplied by p, q, or r, where0<r<q<p<1. When three or more middle contention windows are set, acorresponding adjustment process is implemented based on the describedprocesses. Thus, a contention window adjustment method of the inventioncan retain higher efficiency in a network.

Although the present invention has been described in terms of preferredembodiment, it is not intended to limit the invention thereto. Thoseskilled in the technology can still make various alterations andmodifications without departing from the scope and spirit of thisinvention. Therefore, the scope of the present invention shall bedefined and protected by the following claims and their equivalents.

1. A contention window adjustment method capable of load-adaptivebackoff (LAB) in a network, comprising: setting a contention window;providing at least one first station and one second station; the firststation obtaining backoff time for a reciprocal operation; when thereciprocal operation is complete, the first station transmitting dataframes; and when the data frames are successfully transmitted, the firststation decreasing the contention window by a ratio parameter.
 2. Thecontention window adjustment method as claimed in claim 1, furthercomprising: when the reciprocal operation is complete, the first stationdetermining whether the second station is transmitting data frames; ifthe second station is transmitting data frames, the first stationsuspending the reciprocal operation; and if the second station does nottransmit any data frames, the first station continuously transmittingthe data frames.
 3. The contention window adjustment method as claimedin claim 2, further comprising: defining the maximum contention window,a minimum contention window, and at least one middle contention window;when the data frames transmission is complete, determining whether thecontention window of the first station is greater than the middlecontention window; if the contention window is greater than the middlecontention window, decreasing the contention window with the first ratioparameter; and if the contention window is less than or equal to themiddle contention window, decreasing the contention window with thesecond ratio parameter.
 4. The contention window adjustment method asclaimed in claim 3, wherein the first ratio parameter is greater thanthe second ratio parameter.
 5. The contention window adjustment methodas claimed in claim 4, wherein the first ratio parameter is 75%.
 6. Thecontention window adjustment method as claimed in claim 4, wherein thesecond ratio parameter is 50%.
 7. The contention window adjustmentmethod as claimed in claim 3, further comprising doubling the contentionwindow of the first station.
 8. A machine-readable storage medium forstoring a computer program providing a contention window adjustmentmethod capable of load-adaptive backoff (LAB) function in a network,comprising using a computer to perform the steps of: setting acontention window; providing at least one first station and one secondstation; the first station obtaining backoff time for a reciprocaloperation; when the reciprocal operation is complete, the first stationtransmitting data frames; and when the data frames are successfullytransmitted, the first station decreasing the contention window with aratio parameter.
 9. The machine-readable storage medium as claimed inclaim 8, further comprising: when the reciprocal operation is complete,the first station determining whether the second station is transmittingdata frames; if the second station is transmitting data frames, thefirst station suspending the reciprocal operation; and if the secondstation does not transmit any data frames, the first stationcontinuously transmitting the data frames.
 10. The machine-readablestorage medium as claimed in claim 9, further comprising: defining themaximum contention window, a minimum contention window, and at least onemiddle contention window; when the data frames transmission is complete,determining whether the contention window of the first station isgreater than the middle contention window; if the contention window isgreater than the middle contention window, decreasing the contentionwindow with the first ratio parameter; and if the contention window isless than or equal to than the middle contention window, decreasing thecontention window with the second ratio parameter.
 11. Themachine-readable storage medium as claimed in claim 10, wherein thefirst ratio parameter is greater than the second ratio parameter. 12.The machine-readable storage medium as claimed in claim 11, wherein thefirst ratio parameter is 75%.
 13. The machine-readable storage medium asclaimed in claim 11, wherein the second ratio parameter is 50%.
 14. Themachine-readable storage medium as claimed in claim 10, furthercomprising doubling the contention window of the first station.